RTMT displays an "LowActivePartitionAvailableDiskSpace alert, which indicates there is a low disk space on active partition
CCM Vertion - 5.1.2
Disk /active free space available - 510388k
Can you kindly update us if you are using CAR/CDR ?
As this can be a well known issue here where in CUCM 5.1.2 scheduled CAR reports are written to the active partition and Having most or all scheduled CAR reports enabled and archived for as long as possible may consume all available disk space on the active partition.
What you can do for the timebeing is stop CDR/CAR if not used or purge CDR records to clear disk space other wise if the consumption increases you will need to open a TAC case so that a TAC engineer can apply a workaround for you.